EP0457604B1 - Impedanz-gesteuerter Defibrillator - Google Patents

Impedanz-gesteuerter Defibrillator Download PDF

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Publication number
EP0457604B1
EP0457604B1 EP91304446A EP91304446A EP0457604B1 EP 0457604 B1 EP0457604 B1 EP 0457604B1 EP 91304446 A EP91304446 A EP 91304446A EP 91304446 A EP91304446 A EP 91304446A EP 0457604 B1 EP0457604 B1 EP 0457604B1
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EP
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Prior art keywords
energy
patient
impedance
storage device
defibrillation
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EP91304446A
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French (fr)
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EP0457604A2 (de
EP0457604A3 (en
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Martin G. Rockwell
Francis M. Charbonnier
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HP Inc
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Hewlett Packard Co
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00—Electrotherapy; Circuits therefor
    • A61N1/18—Applying electric currents by contact electrodes
    • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
    • A61N1/38—Applying electric currents by contact electrodes alternating or intermittent currents for producing shock effects
    • A61N1/39—Heart defibrillators
    • A61N1/3925—Monitoring; Protecting
    • A61N1/3937—Monitoring output parameters

Definitions

  • the present invention relates generally to the field of cardiac defibrillation, and in particular to a defibrillation apparatus that optimizes the probability of successful and safe defibrillation on the first attempt.
  • a defibrillator is a device used to administer a high intensity electrical shock through a pair of electrodes, or "paddles,” to the chest of a patient in cardiac arrest.
  • a selected, discrete quantity of energy is typically stored in a capacitor and is then electrically discharged into the patient through the paddle circuit.
  • Defibrillation is not a procedure with a certain and successful outcome. Rather, the probability of successful defibrillation depends on the condition of the patient and on the defibrillation discharge parameters. In order to practice defibrillation successfully and safely, it is important to quickly make an optimum choice of the defibrillation discharge intensity level. If the selected discharge intensity level is too low, defibrillation will not be successful and must be repeated at a higher intensity level until the patient is defibrillated. However, repeated defibrillation discharges at increasing intensity levels are more likely to cause damage to the heart. Also, repeated discharges cause the patient to remain in ventricular fibrillation for a longer time.
  • intensity has been purposely used here to describe the level of the discharge. Intensity may be measured by any one of several discharge parameters, such as energy stored in the defibrillator, energy delivered to the patient, peak or average current flowing through the patient, electrical charge or integrated electron flow through the patient, etc.
  • discharge parameters such as energy stored in the defibrillator, energy delivered to the patient, peak or average current flowing through the patient, electrical charge or integrated electron flow through the patient, etc.
  • defibrillators have been designed to allow control and selection of the energy E s stored in the defibrillator capacitor. This is equivalent to selection of the energy E d (50) which will be delivered into a 50 ohm load, i.e., delivered into the patient if a patient transthoracic impedance (Z p ) of 50 ohms is assumed.
  • all defibrillators currently on the market are designed to permit the operator to select E d (50) only.
  • generally accepted protocols for defibrillation such as that published by the American Heart Association in the Journal of the American Medical Association ("Standards and Guidelines for Cardiopulmonary Resuscitation and Energy Cardiac Care," JAMA, Vol.
  • Delivered energy is not a satisfactory parameter to control defibrillator success and safety, since both the energy required for defibrillation success and the energy threshold for damage to the heart depend strongly on the patient's Z p . That is, a patient with a very low Z p (e.g., 25 ohms) may be successfully defibrillated by an energy of only 50 Joules and may suffer heart damage from an energy as low as 200 Joules, whereas a patient with a high Z p (e.g., 100 ohms) may require 300 Joules for successful defibrillation and may suffer damage only at much higher energies, e.g., 500 Joules or more.
  • the low-to-average Z p group had an average defibrillation threshold of 135 Joules (delivered energy) and 29 amperes (peak current), whereas the high Z p group had an average threshold of 211 Joules and 28 amperes.
  • the defibrillation threshold measured by the peak current was the same for all patients irrespective of Z p
  • the defibrillation threshold measured by the delivered energy did increase with Z p .
  • This parameter value is universal in that it is essentially independent of patient transthoracic impedance and of discharge pulse waveform morphology and duration (whereas the optimum value of I a or Q depend on pulse duration, that of I m depends on both waveform morphology and pulse duration, and that of delivered energy depends on patient Z p ).
  • This universal parameter is the impedance-corrected delivered energy E d /Z p in Joules per ohm.
  • E d /Z p shows the observed defibrillation success rate as a function of E d /Z p for the current multicenter study.
  • a minimum value E d /Z p of approximately 3 Joules per ohm is required to achieve a high probability (70%) of successful defibrillation.
  • the highest success probability (87%) is achieved for values of E d /Z p in the range of 3.75 to 4.50.
  • this protocol has limited efficacy because patient Z p is highly variable and the recommended initial level of 200 Joules is too low for patients with high Z p (e.g., 100 ohms) and is excessive (i.e., unnecessary risk of heart damage) for patients with low Z p (e.g., 25 ohms).
  • a defibrillator for providing a defibrillation pulse to a patient to restore normal cardiac rhythm, comprising: an energy storage device; impedance correction means for establishing a desired value of impedance-corrected delivered energy E d /Z p ; means for measuring a transthoracic impedance Z p of the patient; means for determining a target amount of energy E d required by the energy storage device to provide the desired value of impedance-corrected delivered energy E d /Z p to a patient having the measured value of transthoracic impedance Z; energizing means for providing the target amount of energy to the energy storage device; and de-energizing means for transferring energy stored in the energy storage device to the patient.
  • the desired value of impedance-corrected delivered energy has a value of 3 Joules per ohm.
  • the energizing means may include means for providing further amounts of energy, the second amount of energy being, for example, 3 Joules per ohm and subsequent amounts being, for example, 4.5 Joules per ohm.
  • E d /Z p An alternate, more aggressive value of E d /Z p would be 4 Joules per ohm for initial defibrillation attempts.
  • Fig. 1 is a schematic block diagram of a defibrillator apparatus according to the present invention.
  • Fig. 2 is an equivalent circuit representation representing the load which a defibrillator capacitor discharges.
  • Fig. 3 is a set probability curves illustrating percentages of successful defibrillation of selected energy, selected current, and selected impedance-normalized delivered energy.
  • Fig. 4 is a flow chart diagram of the defibrillator operation.
  • Fig. 5 is a graph showing defibrillation success vs E d /Z p -155 defibrillation attempts.
  • the present invention is an improved defibrillator.
  • the apparatus of the invention is similar to that employed with prior art defibrillation techniques, such as impedance-based current defibrillation.
  • Such apparatuses are shown, inter alia , in U.S. Patents 4,840,177, 4,771,781 and 4,574,810, the disclosures of which are incorporated herein by reference.
  • the present invention is illustrated with reference to an apparatus 10 similar to that shown in U.S. Patent 4,771,781, with some differences noted in the following description.
  • the defibrillation apparatus 10 includes an energy storage device 12, such as a capacitor, paddles 14 for applying an electrical shock from the capacitor to a patient 16, and a power supply 18 for charching the capacitor device.
  • a detector 20 detects the charge accumulated by the capacitor and feeds this data back to a comparator/charge control circuit 22, which controls the capacitor's charging accordingly. Charging of the capacitor continues until the voltage across the capacitor reaches a target value, detailed below.
  • the defibrillator 10 also includes an impedance measurement system 24 that excites the patient 16 with a low current A.C. excitation signal through paddles 14 and produces a corresponding analog output signal inversely proportional to the patient's transthoracic impedance.
  • the excitation signal has a frequency of 31 kilohertz, a frequency at which the patient's transthoracic impedance has been found to approximate the patient's impedance to a defibrillation pulse. In alternate embodiments, however, other excitation frequencies may be used.
  • the output signal from the impedance measurement system 24 is provided to an eight bit analog-to-digital converter 26, such as a National Semiconductor type ADC-0844, which periodically samples the analog signal and converts each sample into digital form.
  • the frequency at which the A/D converter 26 samples is dictated by an associated single board computer 28 and is approximately 240 hertz in an illustrative embodiment.
  • Computer 28 may be built around an Intel 8052 microprocessor with associated ROM and is used to implement many of the subsequent processing functions illustrated in Fig. 1.
  • the A/D sampling continues during a predetermined measurement period, here termed the patient contact interval (PCI), the length of which is set by a software implemented decrementing timer 30 in the computer 28. This period may be approximately one second, a period long enough to yield approximately 240 impedance samples.
  • PCI patient contact interval
  • the samples output from the A/D converter 26 are provided to the single board computer 28 for further processing.
  • the first processing step is to low pass filter the digitized voltage using a software implemented filter 32 to remove noise that may be impressed on this signal.
  • the filtered, digitized voltage signal is then used to index a first look-up table 34 that correlates the output from the A/D converter 26 to a value of patient transthoracic impedance.
  • the first look-up table 34, together with a second look-up table 40 illustrated in Fig. 1, are implemented using the ROM memory associated with the single board computer 28.
  • Each transthoracic impedance value retrieved from the first look-up table 34 is compared by a minimum impedance detector 36 against a minimum impedance value Z min stored in a register 38 in the microprocessor.
  • This register is initially loaded with a value of 255 ohms (FF hex). Thereafter, as each of the impedance values is retrieved from the look-up table 34, it is compared with the register value. If a sampled impedance value is lower than the previously stored minimum, the register is loaded with the new minimum value. At the end of the one second PCI interval, this Z min register contains the lowest impedance value sampled during the interval. This value, here termed Z p , is the patient's transthoracic impedance.
  • the Z p value is used as a first index into a second look-up table memory 40.
  • the desired impedance-normalized delivered energy (E d /Z p ) is used as a second index into the table.
  • This latter figure is typically provided from the computer 28 according to a predefined protocol. (In the illustrated embodiment, this protocol specifies a figure of 3 Joules/Ohm for the first and second successive defibrillation attempts, and a figure of 5 Joules/Ohm for the third and successive attempts.)
  • the impedance-normalized delivered energy may be selected by the operator using a switch 39 on the front panel of the defibrillator.
  • the output from the second look-up table 40 in response to these two inputs is the voltage V to which the capacitor should be charged to deliver the desired value of E d /Z p .
  • This target voltage value V is stored in a register 44 associated with the computer 28.
  • Fig. 2 shows a simplified equivalent patient/defibrillator circuit with which operation of the second look-up table 40 may be more readily understood.
  • the defibrillator capacitor 12 discharges into a series circuit composed of the patients's transthoracic impedance Z p and a defibrillator internal resistance R j (this latter resistance is associated with a series inductor, not shown).
  • R j this latter resistance is associated with a series inductor, not shown.
  • E d E s Z p /(Z p + R j ) (1) If an impedance-normalized delivered energy (E d /Z p ) of 3 joules/Ohm is desired, the total delivered energy E d is simply 3Z p Joules.
  • the values R j and C are invariant and depend on the defibrillator's particular design.
  • R i is typically in the range or 9 to 13 ohms for most defibrillators.
  • the second look-up table 40 is organized as a two-dimensional array indexed by these two variables and contains the corresponding results of equation (4), indicated by V CIRCUIT in Fig. 1.
  • the capacitor 12 is charged by the power supply 18 to the voltage V CIRCUIT (also referenced herein as the "limit” or “target” voltage) obtained from the second look-up table 40.
  • the power supply 18 can comprise a conventional flyback charger in which a 14 volt DC battery is applied to one end of a transformer winding and the bottom end of that winding gates on and off to ground. Suitable chargers are described in U.S. Patents 4,233,659 and 4,119,903.
  • the voltage on the capacitor 12 is sampled through a 1000 to 1 voltage divider network 20 and is converted into digital form by a second analog-to-digital converter 42.
  • the second ADC 42 is actually a second channel of the National Semiconductor ADC 0844 employed as the first ADC 26.
  • the output from the second ADC 42 is compared by the computer with the target voltage V stored in the register 41.
  • the computer 28 allows the power supply 18 to continue charging the capacitor until this target voltage is reached.
  • the defibrillator charge is discharged into the patient by known techniques.
  • This process can be modified to accommodate the common practice of defibrillator operators who often will initiate charge, delay placing the paddles firmly on the patient until the charge is complete or nearly complete, then expect to discharge very quickly. Since patient impedance can be measured only when the paddles are in contact with a patient, the desired capacitor voltage can only be known after the paddles are on the patient, and it is desirable to reach this voltage very quickly (a fraction of a second) after the paddles are placed on the patient. Due to inherent limitations in the power supply, the capacitor voltage can be increased only slowly, whereas it can be decreased very rapidly by partial discharge into an internal resistive load.
  • This feature may be used in all defibrillation protocols which require determination of Z p to set the desired discharge level. Examples of such protocols are current based defibrillation (as disclosed in patents 4,840,177 and 4,771,781) and the impedance-based defibrillation discussed here.
  • the impedance-normalized delivered energy parameter on which the present apparatus is based provides significant advantages over prior art approaches. These advantages are illustrated in Figs. 3A-3C, which represent curves plotting probability of defibrillation success and of damage to the myocardium as a function of selected energy, selected current and selected impedance-normalized delivered energy, respectively.
  • the curves for selected energy (Fig. 3A) are supported by a great deal of published data and are widely known and accepted in their general form.
  • the curves for selected current (Fig. 3B) are supported by a smaller amount of published data but are known and accepted by medical practitioners expert in defibrillation.
  • the curves for impedance-normalized energy Fig.
  • Fig. 3A represent the selected energy approach. This technique is the one currently recommended by the American Heart Association and specifies that defibrillation first be attempted with an energy of 200 Joules, followed by a second attempt at 200 Joules and further attempts at 300 - 360 Joules. The approximate position of these energy values is illustrated in Fig. 3A.
  • Fig. 3B represent the selected current approach. This is the technique adopted in systems according to U.S. Patent 4,840,177 and involves measuring the patient's transthoracic impedance and charging the defibrillator capacitor to the level required to induce a desired current to flow through the patient on discharge.
  • Fig. 3C represent the approach adopted by the present invention: impedance-normalized delivered energy.
  • the defibrillation success curve has a steeper rise and higher maximum value than in Figs. 3A and 3B.
  • the reason behind this optimized response is that transthoracic impedance and discharge waveform are not left as variables. The patient transthoracic impedance is measured and the capacitor charge is set accordingly.
  • the waveform shape and duration are not factors because energy is used as the relevant parameter instead of current. That is, whether a defibrillator has a 2 millisecond discharge pulse or a 8 millisecond discharge pulse doesn't matter because for a given patient the delivered energy will be the same. (This same delivered energy will be manifested as currents of different magnitude in roughly inverse proportion to the discharge durations.)
  • Fig. 4 is a flow chart detailing the various sequences of steps executed by the defibrillator of the present invention.
  • capacitor charge control 22 and voltage limit register 44 are reset, resetting the capacitor limit voltage stored in voltage limit register 44 to zero.
  • selection switch 39 is operated to initiate circuit operation, an initial Z pmin value of 255 ohms is loaded in minimum impedance register 38 and PCI contact interval timer 30 is initialized to a one second time period.
  • the desired value of E d /Z p selected on switch 39 is sent to strip chart recorder 46 for display.
  • the process proceeds to block B to check whether the final voltage limit has yet been determined. If it has not, the process proceeds to block C to examine whether the one second PCI interval has yet expired. If it has not, PCI interval timer 30 is decremented and a new transthoracic impedance sample is taken. The resultant impedance value provided by first look-up table 34 is compared against the previously stored minimum impedance value Z pmin in block D. If it is less than the previously stored minimum value, the old minimum value is replaced by the new value and the processing loop repeats, comparing the capacitor voltage with the limit voltage.
  • the above-described loop repeats itself at a rate of approximately 240 hertz until the one second PCI timer 30 has timed out.
  • Z pmin is entered into second look-up table 40 which determines the final limit voltage V f . This final limit voltage is substituted in voltage limit register 44 which previously contained the initial limit voltage.
  • the process then checks, in block F, whether the voltage on capacitor 12 is greater than the final limit voltage which has just been determined. If so, the capacitor is connected through relay contacts 48 to discharge load 50 to bleed off the excess charge quickly. When the charge bleeds down to the desired final voltage, the process returns to block A.
  • block A the voltage on the capacitor is again examined. This time through the loop, the final voltage limit has been determined, so block B causes the process to loop indefinitely, comparing the voltage on the capacitor with the final limit voltage until the limit voltage is reached. At this point, charging of capacitor 12 is discontinued.
  • Z pmin is greater than 150 ohms, one assumes that the patient impedance has not been properly measured, possibly because the paddles were not applied to the patient.
  • the PCI interval timer is reinitialized and the sequence of operations in blocks A through G is resumed.
  • the process escapes from the charging loop at block G.
  • the apparatus sets a ready indicator and awaits a discharge request. If any charge is lost from the capacitor due to the capacitor's internal leakage resistance while awaiting a discharge request, a trickle charge circuit (not shown) restores the lost charge.
  • impedance-normalized delivered energy (in Joules per ohm) is a control parameter that can be used advantageously as a predictor of successful defibrillation.
  • This parameter value is essentially independent of patient transthoracic impedance and of discharge pulse waveform (i.e. it has the same value for all patients and all defibrillators), a property not shared by any of the other control parameters which have heretofore been used or proposed.
  • Impedance-normalized delivered energy is thus suitable as a standard on which a universal defibrillation protocol may be based.

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  • Health & Medical Sciences (AREA)
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  • Heart & Thoracic Surgery (AREA)
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  • Biomedical Technology (AREA)
  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
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Claims (9)

  1. Ein Defibrillator (10) zum Zuführen eines Defibrillations-Impulses zu einem Patienten, um den normalen Herzrhythmus wiederherzustellen, der folgende Merkmale aufweist:
    eine Energiespeichervorrichtung (12);
    eine Impedanzkorrektureinrichtung (39) zum Einrichten eines gewünschten Wertes von Impedanz-korrigierter gelieferter Energie Ed/Zp;
    eine Einrichtung zum Messen einer transthorakalen Impedanz Zp des Patienten;
    eine Einrichtung zum Bestimmen eines Zielenergiebetrags Ed, der für die Energiespeichervorrichtung erforderlich ist, um den gewünschten Wert der Impedanz-korrigierten gelieferten Energie Ed/ZP zu einem Patienten zu liefern, der den gemessenen Wert der transthorakalen Impedanz Zp besitzt;
    eine Anregungseinrichtung zum Liefern des Zielenergiebetrags zu der Energiespeichervorrichtung; und
    eine Entregungseinrichtung zum Übertragen von Energie, die in der Energiespeichervorrichtung gespeichert ist, zu dem Patienten.
  2. Ein Defibrillator gemäß Anspruch 1, bei dem
    die Anregungseinrichtung ferner eine Einrichtung zum Liefern eines zweiten Energiebetrages zusätzlich zu dem Zielbetrag zu der Energiespeichervorrichtung aufweist;
    die Entregungseinrichtung ferner eine Einrichtung aufweist, um Energie aus der Energiespeichervorrichtung zu entfernen, bis der Zielpegel erreicht ist, bevor die gespeicherte Energie zu dem Patienten übertragen wird.
  3. Ein Defibrillator gemäß Anspruch 2, bei dem
    die Anregungseinrichtung ferner eine Einrichtung zum Liefern eines dritten Energiebetrages zusätzlich zu dem zweiten Energiebetrag zu der Energiespeichervorrichtung, bevor die transthorakale Impedanz des Patienten gemessen wird, aufweist.
  4. Ein Defibrillator gemäß einem der Ansprüche 1 bis 3, bei dem die Impedanzkorrektureinrichtung den gewünschten Wert der Impedanz-korrigierten gelieferten Energie Ed/Zp ohne Bezug auf spezielle Charakteristika des Patienten einrichtet.
  5. Ein Defibrillator gemäß Anspruch 4, bei dem der gewünschte Wert der Impedanz-korrigierten gelieferten Energie 3 Joule/Ohm ist.
  6. Ein Defibrillator gemäß einem beliebigen der Ansprüche 1 bis 5, bei dem die Anregungseinrichtung ferner eine Einrichtung zum Wiederholen des Lieferns des Zielenergiebetrags zu der Energiespeichervorrichtung aufweist, und die Entregungseinrichtung ferner eine Einrichtung zum Wiederholen des Übertragens der Energie von der Energiespeichereinrichtung aufweist, wenn der normale Herzrhythmus des Patienten durch die erste Übertragung von Energie nicht wiederhergestellt ist.
  7. Ein Defibrillator gemäß Anspruch 6, bei dem der Zielenergiebetrag für das wiederholte Liefern von Energie zu der Energiespeichervorrichtung 5 Joule/Ohm beträgt.
  8. Ein Defibrillator gemäß einem beliebigen der Ansprüche 1 bis 7, der ferner eine Einrichtung zum Messen der Energiespeichereinrichtung aufweist, um den Wert eines physikalisch meßbaren Parameters zu ermitteln, der erforderlich ist, um den Zielenergiebetrag in der Energiespeichervorrichtung zu speichern; und bei dem
    die Anregungs- und die Entregungs-Einrichtung ferner eine Einrichtung zum Zu- und Weg-Führen von Energie zu und von der Energiespeichervorrichtung aufweisen, bis der physikalisch meßbare Parameter dem Zielenergiebetrag gleicht.
  9. Ein Defibrillator (10) zum Liefern eines Defibrillations-Impulses zu einem Patienten, um den normalen Herzrhythmus wiederherzustellen, der folgende Merkmale aufweist:
    einen Kondensator (12);
    eine Einrichtung (39) zum Einrichten eines gewünschten Wertes von Impedanz-korrigierter gelieferter Energie Ed/Zp;
    eine Einrichtung (24) zum Messen einer transthorakalen Impedanz Zp des Patienten;
    eine Einrichtung (40) zum Bestimmen der Spannung, auf die der Kondensator geladen werden muß, um den gewünschten Wert der Impedanz-korrigierten gelieferten Energie Ed/Zp zu einem Patienten zu liefern, der die gemessene transthorakale Impedanz Zp besitzt;
    eine Einrichtung (18, 20, 22, 42, 48, 50) zum Zu- und Weg-Führen einer Ladung zu und von dem Kondensator, bis die Spannung über denselben der berechneten Spannung gleicht; und
    eine Einrichtung (54) zum Entladen des geladenen Kondensators durch den Patienten.
EP91304446A 1990-05-18 1991-05-17 Impedanz-gesteuerter Defibrillator Expired - Lifetime EP0457604B1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US07/526,412 US5111813A (en) 1990-05-18 1990-05-18 Defibrillation employing an impedance-corrected delivered energy
US526412 1990-05-18

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EP0457604A2 EP0457604A2 (de) 1991-11-21
EP0457604A3 EP0457604A3 (en) 1992-03-04
EP0457604B1 true EP0457604B1 (de) 1995-03-29

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US (1) US5111813A (de)
EP (1) EP0457604B1 (de)
JP (1) JP3497178B2 (de)
DE (1) DE69108445T2 (de)
HK (1) HK152195A (de)

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Also Published As

Publication number Publication date
EP0457604A2 (de) 1991-11-21
DE69108445T2 (de) 1995-07-27
JPH06315540A (ja) 1994-11-15
EP0457604A3 (en) 1992-03-04
DE69108445D1 (de) 1995-05-04
JP3497178B2 (ja) 2004-02-16
US5111813A (en) 1992-05-12
HK152195A (en) 1995-09-29

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